In the technology of founding the need has arisen for a source of sand having accurately determined proportions of solid and liquid additives, the source to deliver sand at a variable rate according to the momentary need of the foundrymen. While the art of mixing per se as a mechanical process is well advanced, the expedients known to me all require means for individually varying the feed rate of each component, when a total supply rate is to be changed, in order to maintain the proportions of a mixture unchanged.